1. Field of the Invention
Technology described herein relates to the surface preparation of germanium tin (GeSn) or silicon germanium tin (SiGeSn) layers for subsequent deposition.
2. Description of the Related Art
Germanium was one of the first materials used for semiconductor applications such as CMOS transistors. Due to vast abundance of silicon compared to germanium, however, silicon has been the overwhelming semiconductor material of choice for CMOS manufacture. As device geometries decline according to Moore's Law, the size of transistor components poses challenges to engineers working to make devices that are smaller, faster, use less power, and generate less heat. For example, as the size of a transistor declines, the channel region of the transistor becomes smaller, and the electronic properties of the channel become less viable, with more resistivity and higher threshold voltages.
Carrier mobility is increased in the silicon channel area by using silicon-germanium stressors embedded in the source/drain areas, which enhances the intrinsic mobility of silicon. For future nodes, however, still higher mobility devices are needed.
Switching to higher mobility materials than silicon, such as germanium for pMOSFETs, has been suggested. However, the mobility of germanium is not superior to strained silicon, unless the germanium is also strained. It has been recently discovered that germanium tin (GeSn) grown on the source drain region has the requisite strain for making a superior germanium pMOSFET channel, which takes advantage of the germanium/GeSn lattice mismatch. GeSn and silicon germanium tin (SiGeSn) also have mobilities still higher than Ge so they can potentially be used in channel applications by themselves.
However, during the formation and subsequent treatment of the GeSn layer, the surface can become oxidized or affected by other impurities, affecting the subsequent deposition of any overlayer. The overlayer materials can include Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, a doped SiGeSn layer, an insulator, or a metal. Unlike silicon surfaces, germanium surfaces are not effectively passivated by oxide formation. The formation of unstable germanium oxides under atmospheric conditions lead to point defects in the surface which can lead to defects in subsequently deposited layers. Thus, there is a need for methods of preparing the surface of GeSn or SiGeSn for subsequent overlayer deposition.